This paper presents a low-cost interface for high-value resistive sensors varying over a wide range, from k/spl Omega/ to G/spl Omega/. The proposed circuit that acts as a "resistance to period converter" is suitable to be interfaced to a microcontroller or a counting device. The behavior of real electronic components that produce estimation errors, is taken into account. Experimental results obtained by the realized prototype show a good resolution and repeatability.

A low-cost interface to high-value resistive sensors varying over a wide range

This paper describes the CMOS circuit design of a sensor for detecting changes of capacitance due, for instance, to the incidence of particles or bubbles on the electrodes. The circuit is based on a simple design originating at the University of California, Berkeley, for measuring crosstalk on integrated circuits. The basic front-end sensor circuit comprises eight MOSFETs and has a sensitivity of 40 mV/fF. A differential amplifier receives the outputs from two sensor circuits each having 20-/spl mu/m square inter-digitated electrodes. The resulting sensitivity of the fabricated sensor is 1 V/fF with a noise level equivalent to 10 aF. Monte Carlo circuit simulations have been used to identify transistor dimensions to yield acceptable yield, and prototype custom silicon chips have been fabricated using a 0.8-/spl mu/m CMOS process. Static and dynamic tests, using polyamide particles as small as 10-/spl mu/m diameter, verify correct operation of the sensors. The sensor is now being developed for application in miniature electrical tomography systems.

Microelectronic capacitance transducer for particle detection

This paper proposes the direct connection of different configurations of resistive sensor bridges to a microcontroller without any intermediate active component. Such a direct interface circuit relies on measuring the discharging time of a RC network that includes the resistances of the sensor bridge. For quarter-, half-, and full-bridge circuits, we combine the discharging times to estimate the fractional resistance change x of the bridge arms. Experimental results for half- and full-bridge circuits emulated by resistors yield a nonlinearity error below 0.3%FSR (full-scale range) for x between 0 and 0.1 and an effective resolution of 11 bit. Measurements on two commercial magnetoresistive sensors yield higher nonlinearity errors: 1.8%FSR for an AMR (Anisotropic Magnetoresistive) sensor and 5.8%FSR for a GMR (Giant Magnetoresistive) sensor, which are mainly due to the nonlinearity of the sensors themselves. Therefore, the nonlinearity of the measurement is limited by the sensors, not by the proposed interface circuit and linearisation algorithm.

Direct interface circuit to linearise resistive sensor bridges

An easily implementable signal conditioning circuit for resistive humidity and temperature sensors is presented. It is based on a relaxation oscillator in which both the frequency and the duty-cycle of the square-wave output signal simultaneously carry information from two different types of sensors. The output frequency is linearly related to the resistive unbalance of an active bridge, whereas the duty-cycle is independently controlled by a thermal sensor for controlling temperature error of the humidity sensor (RH). The design, analysis, and experimental characterization of the circuit and its application to a sol-gel thin film porous γ-Al2O3-based humidity sensor and resistance temperature detector are reported. Experimental results confirm the theoretical value predicted. The circuit covering wide resistance measurement range has the potential for remotely monitoring measurement parameters accurately.

Relaxation Oscillator-Based Active Bridge Circuit for Linearly Converting Resistance to Frequency of Resistive Sensor

This paper analyzes the effects of power-supply interference on direct sensor-to-microcontroller interfaces based on measuring the charging/discharging time of an RC circuit that includes the sensor. Power-supply interference becomes a source of trigger noise that affects the measurement of the charging/discharging time and, hence, the measurement of the sensor resistance or capacitance. The measurement uncertainty increases proportionally to the amplitude of the interference and depends on its frequency. Such interference effects are smaller when measuring the discharging time and can be further reduced by placing an additional resistor in the interface circuit. Experimental data obtained from a PIC microcontroller agree with the theoretical predictions

Analysis of Power-Supply Interference Effects on Direct Sensor-to-Microcontroller Interfaces

A signal conditioning circuit for resistive sensors is presented. It is based on a relaxation oscillator in which both the frequency and the duty-cycle of the square-wave output signal simultaneously carry information from a pair of different sensors. The output frequency is linearly related to the resistive unbalance of a Wheatstone bridge, while the duty-cycle is independently controlled by a second sensor. The latter can be a thermoresistor used for thermal compensation of the primary sensor. The design, analysis, and experimental characterization of the circuit and its application to a thick-film pressure sensor are reported. A method for compensating the accuracy degradation caused by the finite switching delays is illustrated, and results on its experimental validation are given.

Oscillator-based signal conditioning with improved linearity for resistive sensors

A signal conditioning circuit based on a relaxation oscillator is presented for use with resistive transducers. Both the frequency and the duty-cycle of the output signal carry independent information coming from a pair of different sensors. Namely, the frequency of the output signal changes linearly with the resistance deviations detected by a Wheatstone bridge, while the duty-cycle is dependent on the resistance of a second sensor.

https://iopscience.iop.org/article/10.1088/0957-0233/8/7/021/pdf

A conditioning circuit for resistive sensors combining frequency and duty-cycle modulation of the same output signal

A new microprocessor-based measurement system using an LC resonance locking technique for capacitance measurement is presented. The measuring circuit automatically finds the resonance frequency and tracks its changes, which depend on capacitance changes. The instrument has high sensitivity and resolution, wide measurement range, fast response time and is insensitive to stray capacitances, due to the shield and the wiring, and to their variations. Although the measuring system can be used in different fields such as laboratory and industrial applications, we designed it as a conditioning circuit for industrial capacitive transducers, such as capacitive pressure or displacement sensors. The circuit showed a response time of the order of 2 ms, a resolution of about 0.1 fF for an 8 pF transducer capacitance, with stray capacitances ranging from 4 to 20 pF, and a sensitivity of about in the range from 6 to 16 pF. The measurement technique and the circuit are described in detail in the paper.

https://iopscience.iop.org/article/10.1088/0957-0233/7/12/014/pdf

A 15 ppm resolution measurement system for capacitance transducers

A signal conditioning circuit for resistive sensors is presented. It is based on a relaxation oscillator in which both the frequency and the duty-cycle of the output signal simultaneously carry information from a pair of different sensors. The output frequency is linearly related to the resistive unbalance of a Wheatstone bridge, while the duty-cycle is independently controlled by a second sensor. For this purpose, a thermoresistor may be used for thermal compensation of the primary sensor. The design, analysis and experimental characterization of the circuit and its application to a thick-film pressure sensor are reported.

C. Ghidini

Papers

Oscillator-based signal conditioning with improved linearity for resistive sensors

A conditioning circuit for resistive sensors combining frequency and duty-cycle modulation of the same output signal

A 15 ppm resolution measurement system for capacitance transducers

Oscillator-based signal conditioning for resistive sensors